The module should be located at .../site_perl/5.8.8/HTML/Tagset.pm.
The module should be loaded using use HTML::Tagset;.

Sounds like
the module is incorrectly located at .../site_perl/5.8.8/HTML/HTML/Tagset.pm, or
the module is incorrectly loaded using use Tagset;.

Adding .../site_perl/5.8.8/HTML to @INC in order to omit HTML:: from the use directive is bound to lead to problems.